The radio spectrum is divided up into bands by frequency and corresponding wavelength. For example, Extremely Low Frequency (ELF) ranges from 3-30 Hz and Super Low Frequency (SLF) ranges from 30-300 Hz. Ultra Low Frequency (ULF) ranges from 300-3000 Hz, Very Low Frequency (VLF) ranges from 3-30 kHz and Low Frequency (LF) ranges from 30-300 kHz. Medium Frequency (MF) ranges from 300-3000 kHz and is common for AM broadcasts. High Frequency (HF) ranges from 3-30 MHz and is common for shortwave and amateur radio broadcasts. Very High Frequency (VHF) ranges from 30-300 MHz and Ultra High Frequency (UHF) ranges from 300-3000 MHz and both are common for television as well as general radio transmission. Super High Frequency (SHF) ranges from 3-30 GHz and Extremely High Frequency (EHF) ranges from 30-300 GHz, and both are applicable to microwave devices.
The UHF and VHF frequency ranges in particular are extremely common for hand held radio transmission and reception. Operable over varying distance bearings depending on geography and power of the transmitter these devices permit one or more parties to communicate verbally. In some situations data transmission may also be performed.
As with normal face to face communication, the duration of a transmission may be extremely short. Often these radios may be used between family or business parties for a wide range of communication needs. Further still, these radios may be utilized in search and rescue operations where the transmitting party has limited or reduced power and/or may be week from injury and therefore unable to maintain contact for long. More concerning, these radios are also frequently employed in clandestine operations. The signal to give a command or indicate the arrival of a party or other event may be extremely brief, yet have profound consequences.
Indeed whether for defense, the location of a lost party, or other purpose, it is often highly desirable to know both when a radio communication occurs and from which direction, i.e., bearing, the signal came.
Several attempts have been made to address the need for a system and method capable of signal detection and bearing determination, especially for UHF and VHF signal transmissions. In general these systems and methods have adopted one of two forms.
In the first case, the system scans a single narrowband receiver/demodulator through all possible signal locations in the spectrum. When a signal is detected in the demodulator, the tuner stops scanning and records the voice channel as well as the signal intercept parameters. The detection receiver sends the intercept frequency information to a second receiver which determines the signal's bearing.
A number of shortcomings are immediately apparent with such a system. First, as there are two receivers, one for detection and one for bearing, a short signal transmission caught by the detection receiver may no longer be in transmission by the time the detection receiver provides the intercept frequency to the bearing receiver.
In addition, a single receiver scanning the spectrum may well miss short duration transmissions that occur elsewhere in the spectrum from where the scan is currently active. Further still, when stopped to record one transmission that has been found, additional transmissions that may be occurring will be missed.
In the second case, the system digitizes samples of the waveform for the entire frequency band of interest. A mathematical operation is then performed to measure all the frequencies that may contain signals. As in the first case, the detection of a signal triggers the activation of a second receiver for bearing determination.
Again, a number of shortcomings exist. As in the first case systems, bearing determination is a separate and subsequent operation performed by a second receiver. For short transmissions there is a high likelihood of the bearing determination failing as the signal transmission may well have ended. In addition, even for multiple signals, the bearing determination occurs on a one by one basis.
Recordings of the detected signal(s) are accomplished by digitally demodulating to a receiver. Power requirements for digital filtering and demodulation are quite high, and cooling of the system components is often also required in warm environments.
Reliant on large components, including the antennas, these systems (first or second case) generally require a good deal of physical real-estate and power supply capability in order to operate. Moreover, systems under the first case and the second case are not easily portable, and typically are quite expensive to install and maintain.
Hence, there is a need for a radio signal detection and bearing system and method that overcomes one or more of the issues and problems identified above.